1. Field of Invention
The present invention relates to a toner for developing electrostatic latent images formed by electrostatic recording image forming methods. More specifically, it relates to a toner for use with noncontact-type hot fusing methods such as flash fusing methods, oven fusing methods and similar methods.
2. Description of the Related Art
In conventional image forming methods, such as electrophotographic methods and the like, there are various types of methods used to fuse a toner image which has been transferred onto a transfer sheet. Such fusing methods include pressure fusing methods which do not use heat, contact-type hot fusing methods such as hot roll fusing methods, and noncontact-type hot fusing methods such as the flash fusing methods and oven fusing methods.
Since toners require different characteristics depending on the fusing method used, the toners must have characteristics suitable for the respective fusing methods. When considering the colorization and high-speed performance required by image forming methods in recent years, the pressure fusing methods are deemed unsuitable due to the limitations of toners. As a result, hot roll fusing methods and flash fusing methods have become the focus of attention.
In hot roll fusing methods, toner image fusion is accomplished by passing a transfer sheet bearing a toner image between a pair of heated rollers. This may result in offset development wherein the toner on the transfer sheet adheres to the heated roller. For this reason, a separation agent such as silicone oil must be applied to the hot roll or an anti-offset agent must be included in the toner.
The flash fusing methods, on the other hand, are methods wherein a toner image carried on a transfer sheet is irradiated by flashes from a discharge tube such as, for example, a xenon flash or similar flash, so as to melt the toner and fuse it to the transfer sheet. Noncontact heat fusing methods, such as the aforesaid flash fusing method and similar methods, do not produce the offset development which occurs in hot roll fusing methods. Offset development does not occur because the toner image on the transfer sheet is melted and fused without contact with a roller.
The use of image forming apparatuses such as copiers and similar devices has increased in recent years. The increasing variety of uses of such apparatuses has likewise increased requirements for image quality. Requirements for image quality include high image density, fine line reproducibility, halftone quality, image texture, and accurate reproducibility relative to generation copies. In addition, particularly excellent fine line reproducibility and halftone quality are required in digital image forming apparatuses.
Among these requirements, fine line reproducibility, halftone quality, image texture, and accurate reproducibility of generation copies are characteristics which are highly dependent on the particle size of the toner. It is proposed that a small size toner should be used having a mean particle size of less than 10 .mu.m. When small size toner is used in noncontact heat fusion methods, there is a tendency toward reduced fusion strength. There is also a tendency toward reduced soiling characteristics (referred to as "smearing" hereinafter) on the surface of the transfer sheets.
Smearing is caused by rubbing together of images as the toner particle size becomes smaller. When smearing is severe, e.g., when forming images such as bar codes and similar images, image quality is reduced by rubbing. This reduces bar code verifiability.
In recent years, there has been a tendency to increase the amount of additives including colorants such as carbon black and similar colorants. These additives reduce toner consumption and increase the opacifying power of the toner, which tends to diminish smear characteristics.
In general, flash-fused toners use carbon black dispersed in a thermoplastic resin by fusion kneading. However, the dispersed state of carbon black is, for example, nonuniform, such that images of superior texture cannot be reproduced due to flocculation of the carbon black during the flash fusion process.
As previously described, it has been proposed to use small size toner having a mean particle size of less than 10 .mu.m because image quality is controlled by toner particle size. However, the various types of toner components must be even more uniformly dispersed with small particle size toners. When the dispersion state is inadequate due to the presence of flocculated carbon black, the toner surface becomes irregular, the charge distribution of the developer becomes broader, the amount of inadequately charged toner increases, and toner flow characteristics deteriorate.
Since reflocculation of the carbon black in the softened toner occurs during the flash fusion process, even the use of smaller size toner does not produce excellent high quality images. That is, when reflocculation of the carbon black occurs during flash fusion, the area without carbon black increases. As a result, when the toner is fused to the sheet in this condition, there is a plurality of small white spots in the image, and defects in fine lines appear, thereby diminishing reproducibility. This condition can be readily viewed when the image is enlarged.